1. Field of the Invention
This invention relates to the directional location of atomic beams, and more particularly to the use of laser beams in the directional location process.
2. Description of the Related Art
It is often necessary to be able to precisely measure the direction of an atomic particle beam, both in the laboratory and other environments. The necessary directional measurement has heretofore been accomplished by sensing the edges of the beam with a quadrant detector, or by inserting a sensing wire in the beam path. A major problem with such measurement techniques, however, is that they interfere with the beam itself, removing atoms from the beam in the process of accomplishing their sensing function. Also, since they actually sense only a small portion of the beam, they operate upon the assumption that the beam will have a given density profile; this assumption may not be totally accurate and can lead to errors. Furthermore, it would be desirable to increase the accuracy of such measurements systems down to the area of 0.1 microradians.
While not directed toward the measurement of beam direction, a recently published article discusses the attainment of an accurate perpendicular alignment between a laser beam and an atomic beam, M. J. Verheijen, H. C. W. Biejerinck, N. F. Verster, "Accurate (0.25 mrad) Perpendicular Alignment of a Continuous-Wave Single-Mode Dye Laser Beam and an Atomic Beam", Review of Scientific Instruments 56(1), January, 1985, pages 62-65. The article discusses the desirability of Doppler-free observations in several types of crossed laser beam-atomic beam experiments, such as observations of the interactions between atoms and photons, state selection or state sensitive detection of the atomic beam, and collision experiments with atoms that are excited by laser beam.
To achieve the desired perpendicularity between the atomic beam and laser beam, the Verheijen article proposes that two anti-parallel (counterpropagating) laser beams of the same frequency be directed through the atomic beam at right angles to the assumed atomic beam axis; the counter-propagating laser beams may be formed by appropriate optical treatment of a beam from a single laser. An indication of misalignment from perpendicularity is obtained for the actual atomic beam direction by first running a single laser beam through the atomic beam, varying the frequency of the single laser beam, measuring the intensity of the laser beam by sensing its fluorescence out of the plane of the laser and atomic beams for each of a number of different frequencies, and performing a computer calculation to determine the centroid of the fluorescence distribution as a function of frequency. A similar process is then followed with the pair of counterpropagating laser beams: the centroid of the excitation profile for the counterpropagating beams as a function of frequency is computer calculated over the same frequency range as with the single laser beam. The centroids of the single and double laser beam profiles are then compared, with any misalignment between the two indicating a variance from perpendicularity between the laser and atomic beams. The angle between the laser beams and atomic beam is then varied by scanning the atomic beam collimator with a stepper motor parallel to the laser beams and perpendicular to the atomic beam, and the centroid positions with one and two laser beams are recalculated at each different atomic beam angle. The position of the atomic beam collimator at which the difference between the centroids equals zero corresponds to accurate perpendicularity between the laser and atomic beams.
While the Verheijen, et al. article discloses an improved technique for aligning a laser beam to an atomic beam, there is no disclosure of a mechanism for determining the aboslute direction of the atomic beam relative to a reference axis. Also, the article involves a computer calculation at several steps in the process of determining the relative alignment between the laser and atomic beams, whereas it would be desirable to have a real-time monitoring of the actual atomic beam direction without the necessity of performing centroid calculations. Furthermore, the Verheijen, et al. approach requires the use of counterpropagating laser beams close to perpendicular to the atomic beams, at which angle there is a second order Doppler broadening of the fluorescence distribution which is significant in relativistic beams.